Integrated Injection Logic (I.sup.2 L), otherwise known as, merged transistor logic (MTL), integrated circuits employ upward injection vertical bipolar transistors and lateral PNP transistors. A problem in the prior art is the poor injection efficiency of the emitter for the upward injecting vertical transistor and the excessive charge storage in its emitter base region. This results in the requirement of a larger collector to compensate for the low injection efficiency and longer switching times due to excessive charge storage.
C. Mulder, et al "High Speed Integrated Injection Logic," IEEE Journal of Solid State Circuits, June 1976, p. 379, discloses the use of a low dose, high energy boron implant to extend the base region to the N+ subemitter to minimize the N- region and thereby lower charge storage in the emitter base junction. However, he does not differentiate between the extrinsic and the intrinsic base regions, and therefore, Mulder, el al does not have the ability to alter the base profile to favor the diffusion of electrons in the intrinsic base without adversely influencing injection and capacitance in the extrinsic base.
Copending U.S. patent application Ser. No. 792,277, assigned to the instant assignee, by F. H. DeLaMoneda, entitled "Process for Fabrication of MTL Cells," discloses an N+ substrate emitter, a p-epi, recessed oxide, n-type lateral base region, n+ collectors with a p-type implant whose doping profile is tailored by the thickness of the oxide/nitride masking layer to have its entire doping profile within the N+ substrate emitter under the intrinsic base and adjacent to the N+ substrate in the extrinsic base. The DeLaMoneda application works well for its intended purpose, and minimizes the emitter charge storage problem by using p-type epi, but this compromise degrades the conventional NPN device. The DeLaMoneda application solves the problem of unwanted minority carrier injection into the extrinsic base by increasing the extrinsic base doping profile. However, it cannot alter the base profile in the intrinsic base in order to aid diffusion of electrons and decrease diffusion capacitance therein, since such a tailoring would cause the lateral PNP to be shorted between its collector and emitter. In addition, self-alignment between the implanted contour and the collector cannot be achieved in the DeLaMoneda application while at the same time tailoring the implant to lie below his N base region 24 since it would be necessary that the N base region 24 be devoid of the passivation layer during the ion-implantation step and subsequently reoxidized in order to passivate that region with his oxide 20 (or 40 in his third embodiment). Thus, a self-aligned upward collector and buried implantation region, which is essential for a dense, high performance device, cannot be achieved by the DeLaMoneda application.